This invention relates to an improved apparatus for painting the side seam on the inner surface of a can body in an automatic can welder. More particularly, this invention relates to a painting apparatus included in a welder of the type wherein a cylindrical member with both ends open is advanced with its longitudinal axis being horizontal and with the parts to form a side seam being at the top, and the overlapping portion to form a side seam is passed between upper and lower electrodes and welded to form a can body with both ends open and having a side seam, and then, the can is advanced horizontally with the side seam being at the top while the inner surface of the side seam is coated with paint.
A conventional painting apparatus of the type contemplated by this invention is hereunder described by reference to FIG. 1 which is a schematic representation of an applicator in an automatic welder wherein a can body A with both ends open is passed between upper and lower electrically conductive rolls (not shown) to form a side seam S by resistance seam welding, and the can then is advanced horizontally, with the side seam being at the top, in the direction indicated by the arrow Z as it is guided by a supporting member 100 while the inner surface of the side seam is coated with paint.
The supporting member 100 has a diameter smaller than that of the can body A and is positioned horizontally so that it supports and guides the can body as it penetrates the latter. The member 100 has applicator means made of a paint vessel 104 and an applicator roll 106 in the vessel as means to paint the side seam S. The paint vessel 104 may be formed by boring the member 100 or it may be formed separately from the member 100. In most cases, the applicator means is positioned close to the farthest end of the member 100 in the direction in which the can body is advanced (as indicated by the arrow Z).
The supporting member 100 is fixed on its left-hand end (not shown), and a roller 122 is provided on the leading end (the right-hand end in FIG. 1). The member 100 is supported, not fixed, by a roller 121 right under the roller 122 to support the latter, and hence, keep the whole of the member 100 (including an applicator drive mechanism to be described hereunder) horizontal.
Above the supporting member 100 is a can body advancing device composed of an endless belt 101 rotating in the direction indicated by the arrow Y and a magnet 102 fixed on the lower travel 103 of the belt. When the can body A comes under the lower travel 103, it is attracted by the magnet 102 and is sent to the position of A' by passing through the supporting member 100 as it remains in contact with the underside of the belt 101. During the travel to the position of A', the inside surface of the side seam S is coated with paint.
The applicator roll 106 rotates in the direction of the arrow X to apply paint to the inner surface of the side seam. The roll 106 is driven by an endless chain 107 which in turn is driven by a fixed sprocket coaxial with a gear 108 which engages a gear (not shown) coaxial with a pulley 109 which is driven by a belt 113 put around pulleys 110 and 111. A resilient member 112 is placed between the pulleys 110 and 111. The member 112 forces the belt 113 toward a lower belt 117 put around pulleys 114 and 115 (positioned beneath the pulleys 110 and 111) and a tension pulley 116. The belts 113 and 117 are then brought into frictional engagement and the belt 113 is driven by the belt 117. The belt 117 is driven by the rotation of a pulley 118 coaxial with the pulley 115, whereas the pulley 118 is driven by a belt 119 put around said pulley 118 and a pulley 120 which is connected to a driving mechanism not shown.
Needless to say, the roller 122 and the applicator roll drive mechanism (107 to 113) are positioned within the passageway of the can body along the supporting member 100 so that they do not interfere with the passage of the can body A. To be more specific, the side portion C of the can body A is passed between the belts 113 and 117 that are held in contact with each other, then is advanced under the paint vessel 104, and is finally passed between the rollers 112 and 121.
The paint vessel 104 is supplied with paint by conventional means, and has a detector in the vessel that detects a change in the paint level. The tip of the detector is indicated at 123.
The applicator system described above has the following disadvantages. The conventional applicator roll used in the apparatus shown in FIG. 1 has an outer periphery with a single shallow groove 136 made therein as shown in FIG. 2. With such a roll, a low-viscosity paint (about 50 sec/25.degree. C. through #4 Ford cup) can be applied to the inner surface of the side seam, but because of the low viscosity, the resulting paint film is very thin. If a high-viscosity paint (about 150-380 sec/25.degree. C. in #4 Ford cup) is used, the paint adheres and remains little to both edge parts of the area to be painted by the applicator roll. To keep highly corrosive foods in a welded can for an extended period of time, the side seam must be covered with a thick protective film.
A thick protective film may be obtained by multi-painting of such a low-viscosity paint as described above, but it is apparently time-consuming. If we want to obtain a desired thick film by a single-application of a paint, we must use a high-viscosity paint of the type described above. But, this is not satisfied by the conventional applicator.